The natural heart functions in a fashion similar to a positive displacement pump. Each of the two pumping chambers in the natural heart has two check valves (an inlet and an outlet valve). The walls of the natural heart are made of contractile muscle that provide the power to pump the blood. Each pumping cycle consists of a filling or diastolic phase of the pumping cycle and an ejection or systolic phase of the pumping cycle. During the filling phase, the muscle fibers making up the walls of the heart relax allowing the chamber they surround to fill with blood. During the ejection phase of the cycle the muscle making up the walls of the heart contracts ejecting a portion of the blood from the chamber. The muscle making up the walls of the heart does not all relax simultaneously during the filling phase and does not all contract simultaneously during the ejection phase of the heart's pumping cycle. Portions relax sequentially during the filing phase and they contract sequentially during the ejection phase in such a way to direct or “milk” the blood through the heart in flow that is generally laminar with controlled levels of shear and other parameters of flow.
Mechanical blood pumps have been developed for use as artificial hearts to replace or assist the natural heart. Present blood pumps fall into two general categories. One category uses a rotary impeller and includes centrifugal pumps and axial flow pumps. The other category is pulsatile pumps that provide a flow pattern that more resembles that of the natural heart. My earlier patent, U.S. Pat. No. 6,579,223, discloses a pump with desirable pulsatile flow. This pump comprises a housing, an extensible bladder within the housing, a driving fluid between the bladder and the housing, and device such as a vacuum pump to cyclically decrease and increase the pressure of the driving fluid to expand and contract the elastic bladder. This pump is designed to milk the blood through the apparatus, by progressively expanding the bladder in the direction of fluid flow and progressively contracting the bladder, also in the direction of flow. The pumping chamber has an inlet and an outlet, each of which is equipped with a one-way flow check valve. The elastic expansion and contraction of the bladder reduces the formation of blood clots (thrombosis) in the pump.
A problem with some prior art blood pumps relates to the pattern of blood flow through the pump, i.e., the flow pattern. Significant turbulence occurs in the pumping chamber during the pumping cycle and fluid flow parameters such as shear may not remain in physiological range. There is little other than changing the dimensions and geometry of the pumping chamber that can be done to control the characteristics of blood flow through the pumping chamber. There are areas of high velocity with potential high shear and other areas of slow flow. The areas of high shear may activate platelets and the activated platelets will then contribute to clot formation in slow flow areas. Turbulence leads to energy loss and inefficiency of the pump. Excessive turbulence can also damage the blood cells. Although my prior patent addresses this issue, further improvements are needed in creating a flow pattern that resembles that of a natural heart. Also, there is a need for a process of designing the pump components, particularly the bladder, to achieve the desired flow pattern.